Abstract. A previously unrecognized type of gas fractionation occurs in firn air
columns subjected to intense convection. It is a form of kinetic
fractionation that depends on the fact that different gases have different
molecular diffusivities. Convective mixing continually disturbs diffusive
equilibrium, and gases diffuse back toward diffusive equilibrium under the
influence of gravity and thermal gradients. In near-surface firn where
convection and diffusion compete as gas transport mechanisms, slow-diffusing
gases such as krypton (Kr) and xenon (Xe) are more heavily impacted by
convection than fast diffusing gases such as nitrogen (N2) and argon (Ar),
and the signals are preserved in deep firn and ice. We show a simple theory
that predicts this kinetic effect, and the theory is confirmed by
observations using a newly-developed Kr and Xe stable isotope system in air
samples from the Megadunes field site on the East Antarctic plateau.
Numerical simulations confirm the effect's magnitude at this site. A main
purpose of this work is to support the development of a proxy indicator of
past convection in firn, for use in ice-core gas records. To this aim, we
also show with the simulations that the magnitude of the kinetic effect is
fairly insensitive to the exact profile of convective strength, if the
overall thickness of the convective zone is kept constant. These results
suggest that it may be feasible to test for the existence of an extremely
deep (~30–40 m) convective zone, which has been hypothesized for
glacial maxima, by future ice-core measurements.